The present invention relates to methods of using photosensitive polyimides as dielectrics in the fabrication of microelectronic devices.
The fabrication of integrated circuit devices and the production of high density interconnect structures utilize photolithographic patterning to provide patterned layers of a dielectric composition. Photolithographic patterning generally includes the steps of: (1) forming a layer of the dielectric composition; (2) exposing the layer to a pattern of light, such as ultraviolet light, that corresponds to the configuration of the dielectric layer desired; and (3) developing away the unexposed portions of the dielectric layer with a developing solvent in which the unexposed portions of the dielectric composition are soluble. For instance, in the fabrication of integrated circuits, photolithographic patterning of a dielectric layer is used to electroplate conductor lines onto a substrate. A layer of a dielectric composition is coated onto the substrate and is photolithographically patterned so that the portions of substrate to be electroplated are uncovered. In the production of high density interconnect structures, photolithographic patterning can be used to provide the discrete connections (i.e., vias) between vertically-spaced conductor lines that are otherwise separated by the layer of the dielectric composition.
Polyimides have been identified as a dielectric composition useful in microelectronic device fabrication because of their low relative dielectric constant, high temperature stability, and planarization properties. Several types of polyimide dielectrics are available, such as photosensitive polyimide compositions prepared from polyamic acid precursors containing photosensitizers, and solvent-soluble pre-imidized photosensitive polyimide compositions. The commercial availability of these photosensitive dielectric compositions has simplified the photolithographic patterning of these dielectrics and improved the reliability of microelectronic devices containing the patterned dielectrics. Unfortunately, the known methods of photographically patterning photosensitive polyimide dielectric compositions suffer from several limitations; for instance, prior to the present invention, methods for photolithographically patterning a layer of the photosensitive polyimide compositions frequently resulted in layers that were less than uniformly irradiated and that exhibited unsatisfactory vertical wall profiles. Also, when polyimide layers prepared from polyamic acid precursors containing photosensitizers are coated directly over metal conductor seed lines, the precursors have a tendency to chemically interact with the metals, such as copper, preventing satisfactory patterning over the underlying metal layer. Other methods of photolithographically patterning photosensitive polyimides also require that the polyimide and underlying microelectronic components be exposed to elevated baking temperatures which limits the types of materials that can be used. For example, at temperatures above about 250.degree. C., adjacent layers of metals that form the conductor seed lines are susceptible to interdiffusion and forming brittle, intermetallic alloys.